How would you go about making a polarizing camera filter in Cycles?
You can fake this effect from the material point of view not from Camera or global as @JaroslavJerrynoNovotny stated in the comments.

This effect can be used to control object Roughness (best with Principled BSDF) or for mixing material/shader with Transparent Shader (water, glass) or just for mixing different materials.

Scene setup



Most important thing here is to have object in front of Camera which will be our "filter".

I've used Plane parented to Camera for ease of use.

Materials

"Filter"

It's only Transparent shader. Pure white value for invisible filter or a little bit darker to make it more realistic as polarization filters tend to be darker.

Water/Glass/Mix



In this setup we are using Light Path > Transparent Depth as a mix factor to utilize our transparent filter. Math node controls how strong this mix is. You can make material fully transparent or leave some reflections as I did.

To make this process faster along other materials in the scene you can create simple group out of Light Path, Math, Transparent and Mix exposing Math socket and shader socket. 





Other materials

You can use Light Path > Transparent Depth (with Math) as a Roughness input but this method seems to be way to artificial. If I come with better idea than this I'll make an edit.